This invention relates to sample preparation equipment and methods, and more particularly to diffractometry sample preparation systems, and most particularly to a front-loading soft press system for preparing X-ray diffraction (XRD) samples in minerals processing industries.
X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and it can provide information on unit cell dimensions. The technique generally involves the steps of providing sample material, finely grinding the sample material, homogenizing the finely-ground sample material, and determining the average bulk composition of the homogenized sample.
The current standard sample preparation technique is known as “back-loading”, wherein sample material is first poured into sample holder having a bottom surface, the sample material is pressed into the sample holder forming a sample material measuring surface adjacent the bottom surface of the sample holder, removing the sample holder, and measuring the sample material at a measuring surface formed by the bottom surface of the sample holder. The bottom surface of the sample holder is textured so as to impart a sample material measuring surface which has defined, repeatable roughness characteristics. Since the measuring surface of the pressed sample is not necessarily affected by direct pressure imparted by a pressing component or by physical surface interactions with pressing components during pressing of the sample material, the phenominon well-known as “preferred orientation” is reduced at the measuring surface of the prepared sample. FIG. 2 is a two-dimensional schematic drawing showing an instance of preferred orientation of fine granules which might be caused by pressing sample material with a glass plate. FIG. 3 is a two-dimensional representation which schematically illustrates the more desireable condition of low preferred orientation, otherwise known as “highly random crystal orientation”. The configuration shown in FIG. 3 may be prepared by dusting a tiny amount of fine sample material particles onto a greased surface. As can be clearly seen from the figures, the dusted sample shown in
FIG. 3 has a much more random orientation than the pressed sample shown in FIG. 2, and the pressed sample of FIG. 2 exhibits a much higher pressed orientation than the dusted sample of FIG. 3.
The illustrative examples shown in FIGS. 2 and 3 are only representative of extremely thin layers which would not properly represent bulk materials such as ore tonnages utilized in mineral processing. Accordingly, such methods are not very suitable for minerals processing industries, but are rather much better suited for industries such as cosmetics and pharmaceuticals (where product consistency is tightly controlled and only very tiny amounts of sample material are required since they are much more representative of the material bulk as a whole).
Preferred orientation (i.e. phase identification) is often viewed negatively if present in XRD samples, as it tends to reduce the accuracy of readings. Such errors may ultimately hinder determinations regarding material composition and make-up and lead to inaccurate conclusions. Moreover, for quantitative analysis, it is generally accepted that the measured peak values of reflections (i.e., reflections of type hk0) correspond to or are otherwise indicative of the quantity of the corresponding crystalline phase. Accordingly, for the pressed sample schematically shown in FIG. 2 (which has a higher preferred orientation), reflections of type hk0 would be very low and diminish based on the level of preferred orientation. Conversely, for the dusted sample schematically shown in FIG. 3 (which has low preferred orientation and a highly random crystal orientation), reflections of type hk0 would be higher and increase based on the level of randomness, thereby allowing more accurate determinations about sample material composition.
Several XRD sample preparation systems are already on the market; however, they fail to address objects of the present invention discussed hereinafter. Of these conventional sample preparation systems on the market, Polysius (ThyssenKrupp) offers the Polab® APM Automatic Sample Preparation Module. The Polab® APM basis module consists of a sample loader for blind sample and main sample, a grinding aid dosing unit, a patented grinding unit, and a pellet press.
Herzog offers the HP-PD6 automated press for use with dry powder having a grain of 90 μm. The HP-PD6 applies a “back-loading” pressing process in order to reduce the possibility of adverse influences on prepared pressed samples. The HP-PD6 comprises six stations for pressing samples into steel rings with an aluminum backing. No binder is used, and the press taps the sample gently into the aluminum ring, which aims to reduce preferred orientation, so that it can be used for XRD analysis. In use, a correct dosage of sample material is placed in a ring, residual sample material is “wiped” off from the ring, the sample material is pressed into the ring, and then the ring is then placed on a conveyor belt.
Essa Australia (now a subsidiary of FLSmidth, the Applicant) offers the semi-Automated XRD Press. The Essa® XRD Press is designed to semi-automate the preparation of pressed powder samples for analysis by XRD. A pneumatic piston forms a lightly pressed sample into a two piece sample holder associated with standard XRD analyzers. A sample ring is loaded onto a table and a press is operated to lower a platen. At such a point in time, sample material is placed into the ring. An excess of sample material is required to form a proper pressed sample. The press is operated again, and the sample material is lightly pressed. Excess material is automatically scraped off and vacuumed away. The sample material holder base plate is manually snapped into place and the press is operated again. The sample holder assembly (complete with sample) is rotated onto a discharge base and lowered for manual removal. The press returns to its original state.
All well known vendors who supply automated press tools designated as XRD presses use “back”-loading techniques and not “front”-loading techniques, in order to maximize accuracy of XRD measurements of the samples. However, any gains in accuracy are offset by low sample preparation throughput. Additionally, the back-loading techniques used by all prior XRD sample preparation apparatus require additional complicated sample hardware, since more elements and steps are required to prepare a sample when the measured surfaces of a pressed sample are formed at the bottom of a mold (i.e., blindly formed). This means that more cleaning is necessary, more consumables are required, and more human intervention is necessary. The aforementioned reduces throughput and increases cost.
Although back-loading techniques are widely-accepted as the industry standard, they can only try to reduce instances of preferred orientation phenomena. It is important to note that back-loading techniques do not completely eliminate the phenomenon of preferred orientation. Rather, trade-offs between sample throughput and the amount of preferred orientation present in samples are made during back-loading sample preparation. Moreover, higher variations in crystallographic planes are noticed with back-loading techniques when analyzed over a phi rotation. This yields signal curves which have sinusoidal patterns and unexplained noise—likely due to manual intervention error. See, for example, FIGS. 26 and 27, which show direct comparisons of 4 identical material samples using back-loading (FIG. 26) sample preparation techniques compared to the novel front-loading techniques discussed herein (FIG. 27). As shown in FIG. 27, both quartz samples 1511, 1512 and a mica sample 1513 prepared according to aspects of the invention demonstrate a more uniform distribution than the quartz samples 1501, 1502 and mica sample 1503 shown in FIG. 26, which were prepared using conventional back-loading techniques. It may be appreciated that the non-uniform distributions shown in the back-loading data of FIG. 26 might be attributed to a combination of varying pressure during back loading and non-homogeneous orientation.
Other disadvantages of manual back-loading techniques, include the ‘human’ factor. In many instances, there will always be too many discrepancies with respect to the reproducibility of collected intensities for samples prepared out of the same material. Differences in results may become even larger, if the samples are prepared at different times and/or by different operators.
Another example of a negative effect attributed to manual back-loading is the formation of preferred oriented domains of crystals in cases of human operators (see graph in FIG. 26). The oriented domain formation is not only limited to minerals that show a strong tendency for preferred orientation, but may also be detectable in other minerals that are harder than the surrounding matrix and might exhibit prominent shapes (e.g. quartz). This formation effect may be completely reduced to a statistical presentation of all phases using the novel apparatus and methods described hereinafter.
Transmission/reflection holders are alternative devices which have also been used to reduce preferred orientation during the preparation of XRD samples—particularly thin films and flat samples. Examples of transmission holders may be found in products manufactured by PANalytical and Bruker. In use, sample material is held between two X-ray films and XRD measurements are performed on the sample using a 2-dimensional (2D) or 3-dimensional (3D) detector. However, such devices are only useful with samples comprised of materials (e.g., organics) which allow X-rays to pass. Unfortunately, materials such as crushed mineral-laden ore used in minerals processing are too dense to allow X-rays to pass and be detected, and they are good absorbers of X-rays. Accordingly, only extremely small quantities of finely-ground ore material can be used in samples for a transmission/reflection holders. As previously mentioned, very small samples of ore are not entirely representative of the bulk material in a mountainside, and therefore, it would be largely impractical to utilize such methods and apparatus in minerals processing industries. Moreover, transmission/reflection holder technologies require intensive manual preparation and are not currently automatable.